Refractories are those materials which can withstand very high temperatures. Although there is no well defined dividing line between refractories and non-refractories, most generally recognized refractories have softening temperatures in excess of 1500.degree. C.
The usefulness of refractories depends upon an ability to maintain the mechanical functions at high temperatures, quite often in contact with corrosive liquids and gases. Frequently, they are employed to line furnaces and high temperature vessels. Refractories are also provided in a variety of physical forms and shapes, and can be comprised of plastics, ramming mixes, gunning mixes, casting mixes, etc. In particular, refractory nitrides are useful as crucibles in the melting of metals, and also as components of superhard cutting tools.
In one method of refractory material synthesis, nitrides are prepared by reacting a metal with nitrogen gas. This method, however, requires high furnace temperatures for long periods of time.
The strong exothermic heat effects of chemical reactions has been employed as a process for synthesizing nitrogen refractory materials. This combustion process, known as the self-propagating high temperature synthesis (SHS), has been utilized by numerous investigators.
Merzhanov, et al., discloses a process for the synthesis of refractory inorganic compounds, such as carbides, nitrides, borides, sulfides, and silicides. As disclosed, refractory inorganic compounds are formed utilizing the SHS process with the direct interaction of two chemical elements, one of which, the fuel (usually a metal), is in the condensed state, while the other, the oxiziding agent (non-metal), is either in a condensed or in a gaseous state. This combustion process is carried out in either constant pressure bombs or in special reactors, and initiated with an igniting device.
Borovinskaya, et al., discloses a similar approach employing the SHS process for the production of various refractory inorganic compounds. This process is directed to the synthesis of titanium nitrides, and employs high nitrogen pressures in the range of about 5 to 4500 atm. Beta solid solutions of nitrogen and titanium of various compositions ranging in Stoichiometry from TiN.sub.0.5 to TiN.sub.0.99 are obtained. In this case, high pressure equipment is required to reach full conversion.
One serious drawback of the SHS process is the low percent conversion of metal to nitride. The high adiabatic temperatures of the process, e.g., 3000.degree. to 4800.degree. C., cause the metal to melt as the combustion front propagates through the material. The molten metal forms an effective barrier to the inward diffusion of the nitrogen gas from outside the material.
Complete conversion is more likely to occur when high pressure N.sub.2 gas is employed, e.g., 100-5,000 atm. Unfortunately, the high pressure equipment complicates the process and significantly reduces the advantage over conventional production methods.
It would be an advancement in the art of refractory inorganic material synthesis to provide a method for synthesizing such materials which would not employ high pressures. It would also be desirable to provide a method of synthesizing inorganic refractory material, resulting in nearly complete combustion of the metal starting material, and which is energy efficient. Of particular importance would be to achieve these goals and form refractory metal nitrides wherein the metal is a transition metal from the groups IIIB, IVB, or a rare earth metal.